1. Field of the Invention
The present invention relates to a method for fabricating a wiring board provided with a passive element and to a wiring board provided with a passive element, in particular to a method for fabricating a wiring board provided with a passive element having an aptitude for improving characteristics of the passive element and to a wiring board provided with a passive element having an aptitude for improving characteristics of the passive element.
2. Description of the Related Art
As portable electronic devices become smaller in size, lighter in weight and thinner in thickness, smaller size chip components, such as L (inductor, coil), C (condenser, capacitor) and R (resistor), have been developed. Furthermore, by burying passive elements such as C and L in a wiring substrate, composite components have been developed. Such integration with a wiring substrate has been actively forwarded particularly in multi-layered ceramic substrates and adopted in, for instance, RF (radio frequency) modules for portable telephones.
A multi-layered ceramic substrate is fabricated by first preparing a necessary number of green sheets (ceramic material sheet before sintering) on which coil patterns and capacitor electrode patterns are formed by, for instance, printing a thick film of a conductive paste, followed by stacking these in a predetermined order and by performing simultaneous thermo-compression bonding and sintering.
Furthermore, a plurality of green sheets having different dielectric constants is prepared, and according to characteristics of a passive component to be incorporated, the green sheets are appropriately selected. This is performed by selecting a low dielectric constant green sheet that can set a high self-resonant frequency and secure a high Q-value for a ceramic material that constitutes an inductor, and a high dielectric constant green sheet for a ceramic layer that forms a capacitor, respectively. By use of such a combination, an LC composite component having higher function may be incorporated.
In the aforementioned technique, ceramics is used for wiring substrate material. It is expected that in future an operating frequency of an RF circuit that is used in a portable telephone may reach 10 to 20 GHz. In view of this, it is important from a viewpoint of product costs and so on that organic resins having lower relative dielectric constants are made available as the substrate material. As things are, when an organic resin that is lower in dielectric constant than the ceramics is used as a substrate material (inter-layer insulating layer), and passive elements such as capacitors and so on are incorporated similarly to the multilayer ceramic substrate, it is supposed that an element area may become larger and desired characteristics may not be obtained.
Furthermore, as to the so-called hybrid multi-layered substrates, there are the following reports. That is, first, according to characteristics of a passive element to be incorporated, a substrate material in which a dielectric material and a magnetic material are mingled with polymer is prepared. Thus obtained substrate material is etched, and thereby a patterned capacitor layer and a coil layer are formed. These are laminated in a predetermined order, and thereby a hybrid multi-layered substrate is obtained.
However, in this technique, the multi-layered substrate may warp due to the difference of characteristics such as thermal expansion coefficient and so on between the substrate materials. In addition, since only one specified kind of passive element may be formed in one layer, a design allowance when the passive elements are arranged is low. That is, it is inappropriate in obtaining a smaller size due to tendency to an increase in the number of layers as a whole.
Furthermore, there is another multi-layered organic resin substrate in which resistive paste, dielectric paste, and conductive paste are sequentially printed to incorporate R, L and C. However, since the paste that can be used in this case is restricted to kinds that allow completing heat treatment at temperatures lower than the heat resistance temperature of an organic resin that is used as an insulating layer, the desired characteristics may not be obtained in some cases.